12 MASS. EXPERIMENT STATION BULLETIN 412 



The powder is applied by dusting the stem and upper roots, using a can that 

 has a perforated cover, such as a flour shaker, or by swabbing and brushing 

 it on bj' hand. It is essential to place a good coating on the stem at about the 

 soil level and the amount of powder which naturally sticks to the stem is a satis- 

 factory dosage. One man can easily treat the plants as rapidly as three or four 

 men can set them by hand in the field. An excess of powder on the roots should 

 be avoided to prevent possible injury, especially in dry soil. Plants grown in 

 flats and cut out with soil blocks are difficult to treat by this method. In order 

 to avoid excess soil on the roots, plants growing in coldframes should not be 

 watered less than twenty-four hours before they are lifted for pre-transplanting 

 treatment with calomel-clay dust. 



Pre-transplanting treatment may also be applied by dipping the stem and 

 roots in a calomel-water suspension (2) prepared by adding 8 ounces of the 

 chemical to 10 gallons of water. At Waltham this treatment gave only 62 per- 

 cent protection in 1936. Calomel suspensions v. ere also prepared by adding 

 various adhesive mixtures such as clay, laundry starch, gum arabic, locust 

 bean gum, water soluble wax, and liquid rubber. The suspensions with clay 

 and starch were the most satisfactory and gave good protection. However, they 

 matted and stuck the roots together so persistently that the roots were burned 

 slightly and the plants were stunted during the early stages of growth in the field. 



When pre-transplanting treatments are applied, the maggots occasionally 

 attack the root stem near the surface of the soil and above the portion of the 

 stem which was treated. These stems are frequently badly scarred by channels 

 and small cavities but little or no injury to the growth or development of the 

 plant has resulted from this type of infestation. 



Pre-transplanting treatments are limited in use. The\' are recommended 

 especially on southern-grown plants and others which are transplanted with 

 little or no soil on the roots and are set out by hand. Dust containing equal parts 

 of calomel and talc or clay has been the most satisfactory material. 



Field Treatments 



The application of protective treatments after the plants are growing in the 

 field is the most common method of control. These treatments may be roughly 

 divided into three types: repellents, toxic dusts, and toxic liquid drenches. 



REPELLENTS 



Tar Paper Discs 



One of the first recommended treatments for combating the cabbage maggot 

 was to place a disc cut from tar paper around the stem of each plant at the sur- 

 face of the soil. These discs were usually about three inches in diameter, hex- 

 agonal in shape, with a slit leading to the center where several flaps permitted the 

 disc to fit tightly around the stem. When the discs fitted tightly and the soil 

 was smoothed so that they lay flat on the soil, they prevented the flies from 

 laying eggs near the plant and the odor of the tar paper repelled them. 



At Waltham in 1933 these discs provided 76 percent commercial protection in 

 a field where 57.33 percent of the untreated cabbage plants were commercially 

 injured. (Table 7). In 1943, when the supply of calomel and corrosive sublimate 

 for agricultural use was limited by war needs, tar paper discs were again used. 



